Character divergence and species co-occurrence

Currently I am using phylogenetic methods to explore patterns of co-occurrence among species in relation to their morphological and ecological characteristics. In general, there are three possibilities. First, co-occurring organisms might tend to have similar ecomorphology, because they are adapted to the same physical environment. Second, co-occurring organisms might tend to have different ecomorphology and niche preferences, thereby reducing competitive interactions and permitting coexistence. The latter situation could arise either as a result of selection for character divergence or because only species with divergent niche preferences are able to colonise overlapping areas, i.e. species sorting. Finally, species differences and patterns of co-occurrence might be unrelated to one another, for example, if random patterns of dispersal determine which species coexist in an area and if measured species differences are not the target of strong selection by the environment or competition. Phylogenetic approaches allow assessment of ecological diversity within an evolutionary framework, and provide a simple null model for exploring the relationship between coexistence and evolutionary divergence.

Regions of unusually high (red) and low (blue) geographical overlap between carnivore sister species, derived from a regression model of overlap against contrast in tooth size.

Using extensive geographic and biological databases for carnivores, I show that morphological divergence in dental traits can predict patterns co-occurrence among closely related species (Davies et al. inpress). One of my future goals is to extent this approach to encompass communities comprised of many interacting species (Davies 2006 Curr. Biol. 16, R645-R647). Preliminary analyses indicate that there is much greater overlap than predicted from morphological divergence in regions where carnivore species richness is highest, for example East Africa. One possible explanation is that the high density of carnivores in this habitat inhibits geographical displacement to competition free space, and that niche space, via shifts in tooth size, is saturated.